Variable volume air conditioning systems are well known in tile industry and are commonly used in different kinds of buildings where zoning is required to maintain desired comfort conditions in a plurality of rooms that are served by a single air handling and air conditioning apparatus. The variable volume system and its variations are described in Chapter 2 of American Society of Heating, Refrigeration and Air Conditioning Engineers Inc. (ASHRAE) handbook; Heating, Ventilating and Air Conditioning Systems and Equipment, 1992 Edition. These systems comprise the following as the essential elements:
an air handling and air conditioning apparatus; PA1 a main supply duct to convey conditioned air to the vicinity of the zones served by the air conditioning apparatus; PA1 a plurality of variable air volume (VAV) boxes comprising dampers and actuators, connected to the main supply duct via branches; PA1 distribution ductwork, connected to downstream of VAV boxes to convey conditioned air to a single or a plurality of air terminals located in each zone; PA1 provision for noise reduction in the form of acoustical duct lining of the distribution ductwork or a silencer downstream of each VAV box; PA1 A plurality of zone thermostats, one for each zone, located in one of the rooms within each zone; and PA1 An automatic control system for each VAV box, which may be pneumatic, electric, electronic or digital electronic, to respond signals from the zone thermostat and to reposition the damper in the VAV box, increasing or decreasing the air volume (air flow rate) into the zone. The control system may also receive and respond to signals from a building automation system. Additionally, it may also control reheat or perimeter heating apparatus. PA1 The first drawback is related to the damper noise. In order to operate at acceptable noise levels, these systems must restrict the pressure drop across the damper within the air terminal (The higher the pressure drop across the damper, the higher the noise). A common method to accomplish this is to control the duct static pressure by means of a separate damper located in a branch duct serving a plurality of such terminals, modulated in response to downstream duct static pressure. The inlet pressure at the terminal is limited to about 60 Pascals. Thus the noise is kept within acceptable limits. Unfortunately, this solution introduces the additional cost and complexity of a pressure regulating damper, a pressure sensor, a control loop and acoustic treatment downstream of the pressure regulating damper. PA1 The second drawback is the difficulty in applying pressure independent controls. Incorporating an air velocity sensor into every branch serving such an air terminal is expensive and impractical, therefore presently systems using this type of terminals do not employ pressure independent controls. Pressure fluctuations at the terminal inlet result in changes in air flow rate, which in turn may cause temperature fluctuations in the room.
The dampers in VAV boxes are generally round or rectangular single blade type, commonly referred to as butterfly dampers. Rectangular multi-blade dampers are used less commonly, due to their higher cost. When throttling to adjust air volume, these dampers reduce pressure by inducing geometric flow separation and turbulence in the air stream. Through lifts mechanism the total pressure is dissipated into heat. In the mean time, turbulence interaction with damper blades and duct walls and pressure perturbations with frequencies within audible range are perceived as noise by the occupants. The noise level, or more precisely the sound power level is a function of damper geometry, blade position, pressure drop across the damper and air flow rate. A method of predicting this damper noise is given in ASHRAE handbook; HVAC applications, 1991 edition. To attenuate this noise, silencers or acoustically lined distribution ductwork are usually employed downstream of the dampers.
To achieve the best possible temperature comfort, ideally each room should be made an independently controlled zone. However, due to the cost and impracticality of using a VAV box, acoustically lined ductwork or a silencer and distribution ductwork for each room, this is rarely done. The usual practice is to lump a number of rooms of similar cooling/heating characteristics into a single zone, and locate the zone thermostat in one of these rooms. However, there are always variations in room loads and occupants'preferences, therefore this grouping does not always work satisfactorily.
To avoid the compromise mentioned above, during the last few years, a different type of air terminal design has been developed, which incorporates a volume control damper within the air terminal itself, thus eliminates the VAV box. Air volume al each terminal can be controlled individually, therefore each room can be made an individual zone by installing a room thermostat to control the damper inside the air terminal. However, there are two major drawbacks associated with this scheme, as follows: